You need to make this change to all of your add-ons.
New Update: Battle for Azeroth expansion has changed the default add-ons directory in the WoW installation folder.
If you did not paste the contents to the correct directory, the add-ons will not show up.
Incorrect extraction: Add-ons need to be extracted to a correct and precise location when they are downloaded.
Corrupt Add-on Directory: The directory where you paste the add-on files might be corrupt or have incorrect configurations which is why the client might not be reading the add-on files correctly.